FIG. 1 depicts a conventional spin tunneling element 10. The conventional spin tunneling element 10 typically resides on a substrate 11 on which seed layer(s) 11 have been formed. The conventional spin tunneling element 10 includes a conventional antiferromagnetic (AFM) layer 14, a conventional pinned layer 16 having a magnetization 17, a conventional barrier layer 18, a conventional free layer 20 having a magnetization 21, and a conventional capping layer 22. In addition, analogous conventional spin tunneling elements (not shown) may include a synthetic free layer, a synthetic pinned layer, or both. A synthetic layer typically includes two ferromagnetic layers antiferromagnetically coupled through a thin conductive layer, such as a Ru layer. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. However, the magnetization 21 of the free layer 20 may move, or switch, in response to an external field.
Such a conventional spin tunneling element 10 can be used as a sensor in tunneling magnetoresistive heads. In such an application, the magnetization 21 of the free layer 20 changes in response to an external field. The change in the magnetization 21 results in a different resistance of the conventional spin tunneling element 10. When the magnetization 21 of the conventional free layer 20 is parallel to the magnetization 17 of the conventional pinned layer 16, the resistance of the conventional spin tunneling element 10 is at a minimum. When the magnetization 21 of the conventional free layer 20 is antiparallel to the magnetization 17 of the conventional pinned layer 16, the resistance of the conventional spin tunneling element 10 is at a maximum. Consequently, the change in the magnetization 21, and thus data in a recording media (not shown), may be determined based on the resistance of the conventional spin tunneling element 10.
To be suitable for use as a sensor in a read head, the conventional spin tunneling 10 is desired to have certain properties. A large percentage change in resistance (ΔR/R) and an appropriate Ra is desired for a large signal. The free layer 20 is desired to be soft, having a coercivity of not more than five Oersted. In addition, a low magnetostriction of λs being not more than 1.0×10−6 (or not less than −1.0×10−6) is desired. In addition, a low interlayer exchange coupling, Hin, of not more than fifty Oersted is desired to help ensure that the magnetization of the free layer 20 is free to respond to an external field.
The conventional spin tunneling element 10 may use crystalline MgO as the conventional barrier layer 18 and CoFeB for the free layer 20. For such conventional spin tunneling elements 10, the high ΔR/R and low Ra may be achieved if the MgO has a [100] texture. As used herein, a specific texture indicates that the layer has a dominant orientation. Thus, the conventional barrier layer 18 of MgO having a [100] texture means that the conventional barrier layer 18 has a dominant [100] orientation. However, for such a conventional spin tunneling element 10, the CoFeB free layer 20 may have poor soft magnetic performance. In particular, the CoFeB free layer 20 may exhibit high magnetostriction, high coercivity, and high anisotropy field. For example, the CoFeB free layer 20 may have a magnetostriction of greater than 4.5×10−6. Consequently, a head using the conventional spin tunneling element 10 may not be sufficiently stable.
Alternatively, the conventional free layer 20 may be a bilayer of CoFeB and NiFe. The NiFe layer is used to improve the soft magnetic performance of the conventional free layer 18. However the use of such a multilayer for the conventional free layer 18 significantly reduces the ΔR/R, and thus the signal. For example, the magnetoresistance may drop from approximately 120% to approximately 45%. When NiFe is added to CoFeB during fabrication, CoFeB is transformed from an amorphous structure to a face-centered cubic (FCC) structure during annealing of the conventional free layer 20. This change in the CoFeB layer results in a lower magnetoresistance. Consequently, the signal in a head using such a conventional free layer 20 is reduced.
Accordingly, what is needed is an improved system and method for providing a spin tunneling element that may be suitable for use in a read head.